Medical substituting element for hard tissues and artificial joint

ABSTRACT

The invention solves a problem associated with a conventional medical substituting element for hard tissues that loosening is likely to occur after the substituting element is mounted on hard tissues, by providing the medical substituting element for hard tissues with: one or a plurality of through hole units into which fixation screws are inserted; a screw stopper for stopping the fixation screw inserted into the through hole unit at the position where the head of the fixation screw becomes lower than the upper end of the through hole unit; and a slip-off prevention member engaging unit formed in the inner wall of the through hole unit at the position higher than the screw stopper, wherein after the fixation screw inserted into the through hole unit is threaded into the hard tissues, a slip-off prevention member engages with the slip-prevention member engaging unit to ensure a slip-off prevention of the fixation screw.

This is a continuation of application Ser. No. 08/625,899, filed Apr. 1,1996, now U.S. Pat. No. 5,879,389, issued Mar. 9, 1999.

TECHNICAL FIELD

The present invention relates to a medical substituting element for hardbody tissues and an artificial joint which are used as a substitute forpart or the whole of the function of damaged hard tissues such as abone, ligament, and tendon, during its treating period orsemipermanently, and more particularly to a medical substituting elementfor hard tissues of the type that the element can be mounted on the hardtissues with fixation screws, and to an artificial joint which ispartially constituted by the element.

BACKGROUND ART

Various medical substituting elements for hard tissues have beendeveloped which are used as a substitute for part or the whole of thefunction of hard tissues such as a bone, ligament, and tendon impairedby an accident or the diseases such as rheumatism or osteoarthritis,during its treatment period or semipermanently.

For example, for medical treatment of a fracture, a fixation metal jig(bone fixation plate) is often used as the medical substituting elementfor hard tissues. This fixation metal jig is made of medical metalmaterials such as stainless steel, cobalt-chrome alloy, titanium, andtitanium alloy. After a fracture is reduced and while the fracture iscured, the fixation metal jig is kept attached to a fracture withfixation screws, the jig traversing the broken position of the bone.

For medical treatment of a damaged ligament or a tendon, reconstructionand reinforcement are carried out using the tendon or fascia, or oftenusing an artificial ligament or tendon. For the reconstruction andreinforcement by an artificial ligament or tendon, one or both ends ofthe artificial ligament or tendon are mounted with a reinforcing membersuch as a device with a boss which is fixed to the bone with fixationscrews to thus fix the artificial ligament or tendon.

For medical treatments of a impaired joint, an artificial joint is oftenused as the medical substituting element for hard tissues. Artificialjoints of various structures developed for specific applications includea hinge type, a hemi-hinge type, a rotating hinge type, a surfacereplacement type, a constraint surface replacement type, a stabilizertype, a meniscus bearing type, an unicompartmental replacement type, atibia plateau type, an non-constraint type, and a hemi-constraint type.

Artificial joints are classified into two types, one having aconstituent mounted on a bone with fixation screws and the other nothaving a constituent mounted on a bone with fixation screws. Forexample, an artificial knee joint of a conventional unicompartmentalreplacement type (a kind of surface replacement types) is of the typenot having a constituent mounted on a bone with fixation screws. Forexample, an artificial knee joint of such an unicompartmentalreplacement type has a structure such as shown in FIG. 8.

FIG. 8(a) is a front view illustrating an artificial knee joint 30 of anunicompartmental replacement type when in use, and FIG. 8(b) is a sideview of the artificial knee joint 30 when in use. The artificial kneejoint 30 (for right knee) shown in FIGS. 8(a) and 8(b) has a tibialcomponent 33 and a femoral component 34, the tibial component 30 beingconstituted by a tibial tray 31 and a tibial bearing insert 32 fixedlypre-mounted on the upper surface (the surface face to the side of thefemoral side when in use) of the tibial tray 31.

The tibial tray 31 has a plate unit 31a, and one long stem 31b and oneshort stem 31c formed on the bottom surface of the plate unit andslightly slanted, respectively made of medical metal materials includingstainless steel such as SUS-316 type, cobalt-chrome-nickel alloy (COP),cobalt-chrome alloy (Vitallium), titanium, titanium alloy, and the like,medical ceramics materials including alumina, zirconia, hydroxyapatite,glass ceramics, and the like, or other medical materials.

The tibial bearing insert 32 is made of high density polyethylene (HDP),ultra-macromolecular polyethylene, or the like, and has a predeterminedcurve surface at its joint surface (the surface in contact with thefemoral component 34 when in use). As described earlier, the tibialbearing insert 32 is fixedly pre-mounted on the upper surface of thetibial tray 31 and functions as a slide member for the femoral component34.

The femoral component 34 has a plate unit 34a curved in a predeterminedshape and two stems 34b formed on this plate unit 34a, respectively madeof material including the above-described medical metal, ceramics, andother materials.

As described earlier, the artificial knee joint 30 is a unicompartmentalreplacement type artificial knee joint, and as shown in FIG. 9substitutes part of the function of the knee joint. In mounting theartificial knee joint 30, the tibial component 33 is mounted on theproximal end of the tibia 40 at a predetermined position, and thefemoral component 34 is mounted on the distal end of the femur 41 at apredetermined position.

In mounting the tibial component 33 on the tibia 40, for example, apredetermined area of the proximal end of the tibia 40 is ground to forma flat surface, two holes for striking the two stems 31b and 31c of thetibial component 33 thereinto are formed in the flat surface area,thereafter the end of the longer stem 31b is first inserted into itshole, the tibial component 33 is rotated using the longer stem 31b as afulcrum, and when the shorter stem 31c reaches its hole, the stems 31band 31c are further struck into the holes. In mounting the tibialcomponent 33, bone cement is also used if necessary.

On the other hand, in mounting the femoral component 34 on the femur 41,for example, after a guide hole, into which the stem 34b of the femoralcomponent 34 is struck, is formed in the distal end of the femur 41 at apredetermined position, the distal end of the femur 41 is ground at apredetermined area and thereafter, the stem 34b is struck into the femur41 along the guide hole. In mounting the femoral component 34, bonecement is also used generally. Reference numeral 42 shown in FIG. 9represents a fibula.

Although a conventional medical substituting element for hard tissues ofa type which is mounted on the hard tissues with fixation screws, isfixed to the hard tissues with fixation screws, a small degree offreedom is practically provided between the substituting element andfixation screws even after the mounting. The reason is that in order notto transmit an impact force applied to the hard tissues directly to themedical substituting element for hard tissues via the fixation screws,the fixation screws are given a capability of changing the direction ofthe impact force to provide a cushioning function. The other reason isthat reverse rotation of the fixation screws is prevented in order forthe medical substituting element for hard tissues not to be moved backoutside. To this end, in a conventional medical substituting element forhard tissues, a through hole unit into which a fixation screw is enteredis not formed with female threads, but only an engaging unit is providedwhich engages with the head of a fixation screw when it is threaded intothe hard tissues to a certain degree, and restricts the furtherthreading.

However, even if a medical substituting element for hard tissues ismounted with a degree of freedom between the element and fixationscrews, vibrations applied to the hard tissues mounted with the medicalsubstituting element transmit directly to the fixation screws, so thatthe fixation screws are spontaneously rotated and moved back outside,causing loosening of the medical substituting element, one of combineddiseases.

Such loosening of a medical substituting element for hard tissues oftenoccurs even for the type having a constituent mounted on a bone withstems struck into the bone, such as the conventional artificial kneejoint of the unicompartmental replacement type shown in FIGS. 8 and 9.This occurrence frequency is conspicuous particularly for a constituentunable to use a long fixation stem, such as a tibial component.

DISCLOSURE OF THE INVENTION

It is a first object of the present invention to provide a medicalsubstituting element for hard tissues to prevent loosening thereof afterit is mounted on hard tissues.

It is a second object of the present invention to provide an artificialjoint to prevent loosening thereof after it is mounted on hard tissues.

A medical substituting element for hard tissues of the present inventionachieving the first object comprises: one or a plurality of through holeunits into which fixation screws are inserted; a screw stopper forstopping the fixation screw inserted into the through hole unit at theposition where the head of the fixation screw becomes lower than theupper end of the through hole unit; and a slip-off prevention memberengaging unit formed in the inner wall of the through hole unit at theposition higher than the screw stopper, wherein after the fixation screwinserted into the through hole unit is threaded into hard tissues, aslip-off prevention unit engages with the slip-prevention memberengaging unit to ensure a slip-off prevention of the fixation screw.

An artificial joint of this invention achieving the second object andhaving a first constituent member mounted on a ground surface formed onone bone constituting a joint and a second constituent member mounted ona ground surface formed on the other bone constituting the joint, thefirst and second constituent members abutting against each otherdirectly or via a slide member, or being coupled together by a memberwith an open/close mechanism, wherein the first or second constituentmember comprises: one or a plurality of through hole units into whichfixation screws are inserted; a screw stopper for stopping the fixationscrew inserted into the through hole unit at the position where the headof the fixation screw becomes lower than the upper end of the throughhole unit; and a slip-off prevention member engaging unit formed in theinner wall of the through hole unit at the position higher than thescrew stopper, wherein after the fixation screw inserted into thethrough hole unit is threaded into hard tissues, a slip-off preventionmember engages with the slip-prevention member engaging unit to ensure aslip-off prevention of the fixation screw.

As described above, with a medical substituting element for hard tissuesand an artificial joint of this invention, the slip-off preventionmember can prevent the fixation screw from spontaneously rotating andmoving back outside after the element is mounted on hard tissues.Therefore, loosening of the medical substituting element for hardtissues is hard to occur, which might otherwise be caused by spontaneousrotation of the fixation screw after the element is mounted on the hardtissues. It is therefore easy to alleviate combined disease by using themedical substituting element for hard tissues and the artificial jointof this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a front view showing an example of an artificial knee jointof a unicompartmental replacement type included in artificial joints ofthe invention, and FIG. 1(b) is a side view of the artificial knee jointof the unicompartmental replacement type shown in FIG. 1(a).

FIG. 2 is a cross sectional view showing an example of a tibial tray ofa unicompartmental type artificial knee joint made of a medicalsubstituting element for hard tissues of this invention, in which aslip-off prevention engaging unit of the tibial tray is shown in anenlarged view.

FIG. 3(a) is a perspective view of a slip-off prevention screw uses asthe slip-off prevention member of the tibial tray shown in FIG. 2, andFIG. 3(b) is a perspective view of a fixation screw of the tibial trayshown in FIG. 2.

FIG. 4 is a front view illustrating a use state of the artificial kneejoint of the unicompartmental replacement type shown in FIG. 1.

FIG. 5 is a plane view showing an example of a stopper ring as one ofslip-off prevention members to be mounted on a medical substitutingelement for hard tissues of this invention.

FIG. 6 is a cross sectional view showing another example of a tibialtray of a unicompartmental type artificial knee joint made of a medicalsubstituting element for hard tissues of this invention, in which aslip-off prevention engaging unit of the tibial tray is shown in anenlarged view.

FIG. 7 is an oblique view showing an example of a caulking projection asone of slip-off prevention members to be mounted on a medicalsubstituting element for hard tissues of this invention.

FIG. 8(a) is a front view showing an example of a conventionalartificial knee joint of a unicompartmental replacement type, and FIG.8(b) is a side view of the artificial knee joint of the unicompartmentalreplacement type shown in FIG. 8(a).

FIG. 9 is a front view illustrating a use state of the artificial kneejoint of the unicompartmental replacement type shown in FIG. 8.

MOST PREFERRED EMBODIMENTS PRACTICING THE INVENTION

The invention will be described in detail.

First, a medical substituting element for hard tissues of this inventionwill be described. This medical substituting element for hard tissues isa medical substituting element fixed to hard tissues by fixation screws.The invention relating to a medical substituting element for hardtissues is applicable to a fixation metal jig (bone fixation plate) forsurgical treatment of a fracture, a an augmentation material (a devicewith a boss) for an artificial ligament or an artificial tendon, anartificial bone which is fixed to a bone with screws, a tibial tray, atibial component and a femoral component of an artificial knee joint, anacetabular component of an artificial hip joint, a tibial component anda talar side component of an artificial ankle joint, an acetabularcomponent of an artificial shoulder, a humeral component and an ulnarcomponent of an artificial elbow, an artificial bone head, and the like.

The overall shape of the medical substituting element for hard tissuesof this invention is not limited in particular, but may be selected asdesired in accordance with each application or the like of a subjectmedical substituting element for hard tissues. For example, if a subjectmedical substituting element for hard tissues is a fixation metal jig(bone fixation plate) for surgical treatment of a fracture, the overallstructure may be rectangular and L-shaped in a side view similar to aconventional shape. If a subject medical substituting element for hardtissues is a an augmentation material (a device with a boss) which ismounted on the end of an artificial ligament of an artificial tendon,this shape may also be a conventional shape. If a subject medicalsubstituting element for hard tissues is a constituent of an artificialjoint, a conventional overall shape may be used although it depends onthe type of a joint to be replaced by the artificial joint and on whichof the two bones constituting the joint the constituent is mounted. Forexample, if a subject medical substituting element for hard tissues is atibial component of an artificial knee joint, its plane view may becircular, oval, of horse saddle, or of other shapes. If a subjectmedical substituting element for hard tissues is a acetabular orglenoidal component of an artificial hip joint or an artificialshoulder, the plane view May be circular, oval, or of other shapes. If asubject medical substituting element for hard tissues is a tibialcomponent or a talus component of an artificial ankle joint, the planeview may be rounded rectangular, rounded trapezoidal, or of othershapes. If a subject medical substituting element for hard tissues is ahumeral component or an ulnar component of an artificial elbow, theplane view may be rectangular, ellipsoidal, circular, or of othershapes. If a subject medical substituting element for hard tissues is aconstituent of an artificial joint as described above, a slide member(e.g., corresponding to the tibial bearing insert 32 of the tibialcomponent 30 shown in FIG. 8) made of polyethylene, ceramics, or thelike may be fixedly pre-mounted or it may be mounted detachably. In thelatter case, an engagement unit for detachably mounting the slide memberis formed if necessary at a predetermined area of the medicalsubstituting element for hard tissues.

The material of the medical substituting element for hard tissues can beselected as desired from medical inorganic solid materials in accordancewith its application. Particular examples of the medical inorganic solidmaterials are medical metal materials including stainless steel such asSUS-316 type, cobalt-chrome-nickel alloy (COP), cobalt-chrome alloy(Vitallium), titanium, titanium alloy, and the like, medical ceramicsmaterials including alumina, zirconia, hydroxyapatite, glass ceramics,sapphire, and the like, bone materials including glass, glass fibers,kiel bones, and the like, macromolecular materials such as syntheticresin and the like including polypropylene, polyethylene, and the like,poly-lactic-acid materials, and other medical materials.

As stated earlier, the medical substituting element for hard tissues ofthis invention has one or a plurality of through hole units into whichfixation screws are inserted. The number of through hole units can beselected as desired depending on the application or the like of themedical substituting element for hard tissues. The thickness of a memberat which the through hole unit is formed is optional so long as aslip-off prevention member engaging unit to be described later can beformed in the inner wall of the through hole unit. If the medicalsubstituting element for hard tissues is a plate unit, a projection isformed on the bottom surface of the plate unit at the position where thethrough hole unit is formed, and the through hole unit is formed passingthrough the projection, so that a space for forming the slip-offprevention engaging unit to be described later can be easily reserved.

Into the through hole unit, a fixation screw is inserted for mountingthe medical substituting element to hard tissues. Although a femalethread portion meshing with the fixation screw is generally unnecessaryto be formed in the through hole unit, it may be formed if necessary. Ascrew stopper is formed in the through hole unit, and stops the insertedfixation screw at the position where the head of the fixation screwbecomes lower than the upper end of the through hole unit. As thefixation screw inserted into the through hole unit continues to bethreaded into the hard tissues, the screw stopper and the head of thefixation screw abut against each other to restrict the threading of thefixation screw. In mounting the medical substituting element for hardtissues, the screw stopper and the head of the fixation screw may beabutted together or may leave a desired amount of gap therebetween.

Formed in the inner wall of the through hole unit at the position higherthan the screw stopper is the slip-off prevention member engaging unitwhich is the most significant feature of the medical substitutingelement for hard tissues of this invention. This slip-off preventionmember engaging unit engages with a slip-off prevention member forpreventing an spontaneous rotation and slip-off of the fixation screwonce threaded into the hard tissues. The configuration of the slip-offprevention member engaging unit is selected as desired in accordancewith the configuration of a member used as the slip-off preventionmember.

Particular examples of the slip-off prevention member to be enumeratedare a screw (hereinafter called a stopper screw) having a threadingrotation direction opposite to the fixation screw, a ring stopper, acaulking projection formed on the head of the fixation screw, and thelike.

If the stopper screw is used as the slip-off prevention member, a femaletread portion meshing with the stopper screw is used as the slip-offprevention engaging unit. For example, if the fixation screw is aright-handed screw, a left-handed screw is used as the slip-offprevention screw. Therefore, the female thread portion meshing with theleft-handed screw (slip-off prevention screw) is used as the slip-offprevention member engaging unit. If the fixation screw is a left-handedscrew, a right-handed screw is used as the slip-off prevention screw.Therefore, the female thread portion meshing with the right-handedscrews (slip-off prevention screw) is used as the slip-off preventionmember engaging unit.

Whether a right- or left-handed screw is used as the fixation screw isdecided as desired, and in correspondence with this decision, whether aleft- or right-handed screw of the female screw portion is formed as theslip-off prevention member engaging unit is decided selectively. Theslip-off prevention screw may be a screw whose head extends out of theupper edge of the through hole unit or may be a screw whose head issubstantially flush with, or lower than, the upper edge of the throughhole unit. If a subject medical substituting element for hard tissues isa constituent of an artificial joint, it is particularly preferable thatthe slip-off prevention screw is a screw whose head is substantiallyflush with, or lower than, the upper edge of the through hole unit.

If a ring stopper is used as the slip-off prevention member, a ringgroove formed in the inner wall of the through hole unit along its innercircumference is used as the slip-off prevention member engaging unit.This groove is sufficient if it has a structure allowing the stopperring used as the slip-off prevention member to engage from the outsidein the radial direction of the stopper ring.

If a caulking projection is used as the slip-off prevention member, asthe slip-off prevention member engaging unit, a ring groove is formed inthe inner wall of the through hole unit along its inner circumference ora recess into which the caulking projection is inserted is formed in theinner wall of the through hole unit. These groove and recess aresufficient if they have a structure allowing the caulking projectioncaulked and used as the slip-off prevention member to engage from theoutside, and are formed near at the central area, rather than the lowerend, of the through hole unit while taking the removal of the caulkingprojection thereafter into consideration.

Although the medical substituting element for hard tissues of thisinvention has the through hole unit, screw stopper, and slip-offprevention member engaging unit, one or a plurality of stems may beformed on the lower surface (the surface in contact with the hardtissues) of the medical substituting element for hard tissues, ifnecessary for each application, aiming at convenience of positionalignment when the medical substituting element for hard tissues ismounted on the hard tissues, or for other objects. In order to mount themedical substituting element for hard tissues with means other than thefixation screws, a central stem, a condylar stem, a fin, a screw, akeel, a hook, a flange formed to cover the anterior or posterior side ofhard tissues (bone), a porous, rough surface, a indented rough surface,a beam, or the like may be formed if necessary.

If the subject medical substituting element for hard tissues is aconstituent (fixed to hard tissues) of an artificial joint of a hingetype or a hemi-hinge type, a hinge axis, a hinge bearing, or the likemay be formed if necessary on the upper surface (the surface opposite tothe surface in contact with the hard tissues) of the medicalsubstituting element for hard tissues.

In mounting the medical substituting element for hard tissues of thisinvention on the hard tissues, the fixation screw is inserted into thethrough hole unit of the medical substituting element for hard tissues,the fixation screw is threaded into the hard tissues until the head ofthe fixation screw is stopped by the screw stopper or until a desiredamount of gap between the head of the fixation screw and the screwstopper is retained, and thereafter the slip-off prevention member isengaged with the above-described slip-off prevention member engagingunit.

Engagement of the slip-off prevention member with the slip-offprevention member engaging unit is performed, if the slip-off preventionmember is a slip-off prevention screw, by meshing the slip-offprevention screw with the female thread as the slip-off preventionmember engaging unit. If the slip-off prevention member is a stopperring, it is performed by engaging the stopper ring with the groove asthe slip-off prevention member engaging unit. If the slip-off preventionmember is a caulking projection formed on the head of the fixationscrew, it is performed by threading the slip-off screw into hard tissuesto a predetermined depth, and thereafter, bending outward (this work iscalled "caulking") the caulking projection formed on the head of thefixation screw to enter the end of the caulking projection into thegroove or recess as the slip-off prevention member engaging unit.

By engaging the slip-off prevention member with the slip-off preventionmember engaging unit in the above manner, it is possible to prevent thespontaneous rotation and slip-off of the fixation screw threaded intothe hard tissues. Accordingly, loosening of the medical substitutingelement for hard tissues is hard to occur, which might otherwise becaused by the spontaneous rotation and slip-off of the fixation screwafter the mount on the hard tissues.

The slip-off prevention member once engaged with the slip-off preventionmember engaging unit can be removed, for example, if the slip-offprevention member is a slip-off prevention screw, by rotating theslip-off prevention screw in a predetermined direction. If the slip-offprevention member is a stopper ring, it can be released by nipping thedesired portion of the stopper ring with a pair of plier or the likewith sharp tips and squeezing them in the direction of reducing itsdiameter to dismount it from the slip-off prevention member engagingunit. If the slip-off prevention member is a caulking projection, it canbe performed by threading the fixation screw further deep into the hardtissues and making it again upright. After the slip-off preventionmember is removed, the fixation screw can be removed easily by rotatingit in the direction opposite to that of threading it into the hardtissues.

Next, the artificial joint of this invention is to be described.

As described previously, the artificial joint of this inventioncomprises a first constituent member mounted on an osteotomized surfaceof the one bone constituting a joint and a second constituent membermounted on an osteotomized surface of the other bone constituting thejoint, the first and second constituent members abutting against eachother directly or via a slide member, or being coupled together by amember with an open/close mechanism. The artificial joint of thisinvention has the features that the first or second constituent membercomprises: one or a plurality of through hole units into which fixationscrews are inserted; a screw stopper for stopping the fixation screwinserted into the through hole unit at the position where the head ofthe fixation screw becomes lower than the upper end of the through holeunit; and a slip-off prevention member engaging unit formed in the innerwall of the through hole unit at the position higher than the screwstopper, wherein after the fixation screw inserted into the through holeunit is threaded into hard tissues, a slip-off prevention member engageswith the slip-prevention member engaging unit to ensure a slip-offprevention of the fixation screw.

Specifically, in the artificial joint of this invention, at least one ofconstituent members constituting the artificial joint is mounted on hardtissues with fixation screws, and the above-described slip-offprevention member engaging unit is formed at a predetermined position ofthe through hole unit of the constituent element into which the fixationscrew is inserted. The other constituent elements and materials of thisconstituent member may be similar to a conventional artificial joint.

The invention relating to an artificial joint is applicable to varioustypes of artificial joints of the type capable of fixation at least oneof the constituent members on hard tissues with fixation screws.Examples of such artificial joints to be enumerated are an artificialknee joint (unicompartmental replacement type, total replacement type,non-constraint type, hemi-constraint type, constraint type, hinge type,hemi-hinge or rotation-hinge type), meniscus bearing type, tibia plateautype, and the like), an artificial hip joint, an artificial joint withan artificial bone, an artificial shoulder joint, an artificial anklejoint, an artificial elbow joint, and the like.

In the artificial joint of this invention, in mounting a constituentmember made of the medical substituting element for hard tissues of thisinvention on the hard tissues, the fixation screw is inserted into thethrough hole unit formed on the constituent member, the fixation screwis threaded into the hard tissues, and thereafter, the slip-offprevention member is engaged with the slip-off prevention memberengaging unit. It is, therefore, possible to prevent the spontaneousrotation and slip-off of the fixation screw once threaded. Accordingly,loosening of the artificial joint is hard to occur, which mightotherwise be caused by the spontaneous rotation and slip-off of thefixation screw after the mount on the hard tissues.

Further, even if a polyethylene slide member (e.g., tibial bearinginsert) is disposed on the constituent member (e.g., tibial tray) fixedto hard tissues with fixation screws, the slide member is hard to bebroken (penetrated and crashed) which might otherwise be caused by thespontaneous rotation and slip-off of the fixation screw.

Furthermore, for a constituent member of the type which is fixed bystriking a fixation stem or the like into hard tissues, it is oftendifficult to elongate the stem or the like because a work space for itsinsertion is limited. However, if the fixation screw is used, this screwcan be easily elongated, and with the longer fixation screw, thefixation strength can be easily increased. Accordingly, with theartificial joint of this invention, it is easy to mount the constituentmember made of the medical substituting element for hard tissues of thisinvention, with better fixation effects, and the surgery is easier thanan artificial joint of the type having no constituent member mounted ona bone with fixation screws. Still further, less invasion alleviates aphysical load on a patient and therefore the recovery after surgery isbetter.

In the artificial joint of this invention, the constituent elementsother than the constituent member made of the medical substitutingelement for hard tissues of this invention can be mounted in the mannersimilar to a conventional manner.

The medical substituting element for hard tissues of this invention hasthe slip-off prevention member engaging unit in the inner wall of thethrough hole unit of the medical substituting element for hard tissuesat its predetermined position. Therefore, the fixation screw is insertedinto the through hole unit and threaded into the hard tissues until thehead of the fixation screw is stopped by the screw stopper or until adesired amount of gap between the head of the fixation screw and thescrew stopper is retained, and thereafter the slip-off prevention memberis engaged with the above-described slip-off prevention member engagingunit. It is therefor possible to prevent the spontaneous rotation andslip-off of the fixation screw once threaded.

If as the slip-off prevention member a slip-off prevention screw havingthe threading rotation direction opposite to the fixation screw is usedand the female thread portion meshing with the slip-off prevention screwis used as the slip-off prevention member engaging unit, and if theslip-off prevention screw is meshed with the female thread portion, theneven if the fixation screw once threaded into the hard tissues intendsto spontaneously rotate and move back outside, a slip-off of thefixation screw is impossible because the rotation direction is that ofthreading the slip-off prevention screw into the hard tissues. Furthersince vibrations of the hard tissues cannot transmit directly to theslip-off prevention screw, it is hard for the slip-off prevention screwto spontaneously rotate and move back outside.

If the ring stopper is used as the slip-off prevention member and thering groove is formed in the inner wall of the through hole unit alongthe inner circumference thereof as the slip-off prevention memberengaging unit to allow it to engage with the ring stopper from theoutside in the radial direction of thereof, then even if the fixationscrew once threaded into the hard tissues intends to spontaneouslyrotate and move back outside, this motion is prevented by the ringstopper so that the fixation screw is impossible to move back outside.

If the caulking projection formed on the head of the fixation screw isused as the slip-off prevention member and the recess or ring groove isformed in the inner wall of the through hole unit to allow the end ofthe caulking projection caulked to engage from the outside, then even ifthe fixation screw once threaded into the hard tissues intends tospontaneously rotate and move back outside, this motion is prevented bythe caulking projection caulked so that the fixation screw is impossibleto move back outside.

As described above, the medical substituting element for hard tissues ofthis invention can prevent the fixation screw from moving back outsideafter the mount on the hard tissues. Therefore, loosening of the medicalsubstituting element for hard tissues is hard to occur, which mightotherwise be caused by the spontaneous rotation and slip-off of thefixation screw after the mount on the hard tissues.

Embodiments of the invention will be described with reference to theaccompanying drawings.

FIG. 1(a) is a front view showing an example of an artificial knee joint1 when in use of a knee of unicompartnmental replacement type includedin artificial joints of the invention, and FIG. 1(b) is a side view ofthe artificial knee joint 1 when in use of the knee of unicompartmentalreplacement type shown in FIG. 1(a).

The artificial knee joint 1 (for right leg) shown in FIGS. 1(a) and 1(b)has a tibial component 4 and a femoral component 5, the tibial component4 being constituted by a tibial tray 2 and a tibial bearing insert 3detachably mounted on the upper surface (the surface on the side of thefemur when in use) of the tibial tray 2.

The tibial tray 2 has a plate unit 2a made of titanium, and twoprojections or sleeves 2b and two stems 2c formed on the bottom surface(the surface in contact with the tibia when in use) of the plate unit2a. The bottom surface of the plate unit 2a is rough and porous. Theprojections 2b and stems 2c are formed to obliquely direct toward theback side of the tibia so that they reach the tibial cortex when thetibial tray 2 is mounted on the tibia. Each projection 2b has a throughhole unit (not shown in FIG. 1) passing through the projection 2b in thelongitudinal direction thereof. A fixation screw 6 is inserted into thisthrough hole unit. An engaging unit (not shown) is formed on the uppersurface of the plate unit 2a, for detachably mounting the tibial bearinginsert 3 on the upper surface of the plate unit 2a. The details of thisthrough hole unit will be later described.

The tibial bearing insert 3 is made of ultra-macromolecularpolyethylene, its bottom surface (the surface in contact with the tibialtray 2 when in use) is formed with an engaging unit (not shown) forengagement with the upper surface of the tibial tray 2, and its topsurface (the surface in contact with the femoral component 5 when inuse) has a predetermined curved surface. This tibial bearing insert 3 ismounted on the top surface of the tibial tray 2 when in use andfunctions as a slide member for the femoral component 5. Since thetibial bearing insert 3 is detachably mounted on the tibial tray 2, ifit is abraded after operation, only this tibial bearing insert 3 can bereplaced.

The femoral component 5 has a plate unit 5a made of cobalt-chrome alloyand curved in a predetermined shape and two stems 5b formed on the uppersurface of this plate unit 5a. The upper surface (the surface in contactwith the femur when in use) of the plate unit 5a is an indented roughsurface.

In the artificial knee joint 1 of the invention of a knee ofunicompartmental replacement type comprising the tibial component withthe tibial tray and tibial bearing insert 3 and the femoral component 5,the tibial tray 2 corresponds to the medical substituting element forhard tissues of this invention.

Specifically, as shown in FIG. 2, each of the two projections 2b formedon the bottom surface of the plate unit 2a constituting the tibial tray2 is formed with a through hole unit 10 passing through the projection2a in the longitudinal direction. As a slip-out prevention memberengagement unit, a female thread portion 12 meshing with a left-handedslip-out prevention screw is formed in the inner wall of each throughhole unit 10. Near at the lower end portion (the end on the insertionside into the tibia) of the through hole unit 10, a screw stopper 13 isformed for stopping the right-handed fixation screw 6 at the positionwhere the head 6a of the fixation screw 6 is lower than the upper end ofthe through hole unit 10, when the fixation screw 6 is inserted into thethrough hole unit 10. The female thread portion 12 is formed in theinner wall of the through hole at the position higher than the screwstopper 13.

The slip-out prevention screw 11 is a left-handed screw as describedabove. Although this slip-out prevention screw 11 has no head as shownin FIG. 3(a), a hexagonal hole 11a is formed in the upper surface of theslip-out prevention screw 11 in order to allow a rod spanner of ahexagonal cross section to enter this hole 11a and rotate the slip-outprevention screw 11.

The fixation screw 6 is a right-handed screw as described above, and hasa flat head as shown in FIG. 3(b). A hexagonal hole 6b is formed in thehead 6a of the fixation screw 6 in order to allow a rod spanner of ahexagonal cross section to enter this hole 6b and rotate the fixationscrew 6.

As described earlier, the artificial knee joint 1 is an unicompartmentalreplacement type artificial knee joint, and as shown in FIG. 4substitutes part of the function of the knee joint. In mounting theartificial knee joint 1 on a knee, the tibial tray 2 is mounted on theproximal end of a tibia 15 at a predetermined position, and the femoralcomponent 5 is mounted on the distal end of the femur 16 at apredetermined position. The tibial bearing insert 3 is mounted on theupper surface of the tibial tray 2.

The tibial tray 2 is mounted on the tibia 15, for example, in thefollowing manner. First, a predetermined area of the proximal end of thetibia 15 is ground to form a flat surface, holes for inserting theprojections 2b and stems 2c of the tibial tray 2 thereinto are formed inthe flat surface area and thereafter, the projections 2b and stems 2care inserted into these holes to align the position of the tibial tray2. Next, each fixation screw 6 is inserted into the through hole unit 6and threaded into the tibia 15, until the head 6a of the fixation screw6 abuts against the screw stopper 13 or until a desired amount of gap isformed between the head 6a of the fixation screw 6 and the screw stopper13, to thereby fix the tibial tray 2 to the tibia 15. Thereafter, theslip-out prevention screw 11 is threaded into and meshed with the femalethread portion 12 to ensure a slip-out prevention of the fixation screw6. A combination of the fixation screw 6, slip-out prevention screw 11,female thread portion 12, and screw stopper 13 is preferably arranged soas to leave a desired amount of gap between the lower surface of theslip-out prevention screw 11 and the top surface of the fixation screw6.

With the slip-out prevention screw 11 being meshed with the femalethread portion 12, even if the fixation screw 6 once threaded into thehard tissues (tibia) is spontaneously rotated and moved back outside,this motion is stopped because the rotation direction is the directionof threading the slip-out prevention screw 11 into the through hole unit10. Accordingly, after the tibial tray 2 is mounted on the tibia 15,loosening of the tibial tray 2 is not likely to be formed by thespontaneous rotation and back motion to outside of the fixation screw 6.For the position alignment of the tibial tray 2, bone cement may be usedif necessary.

On the other hand, in mounting the femoral component 5 on the femur 16,for example, after a guide hole into which the stem 34b of the femoralcomponent 5 is struck is formed in the distal end of the femur 16 at apredetermined position, the distal end of the femur 41 is ground at apredetermined area and thereafter, the stem 5b is struck into the femur16 along the guide hole. In mounting the femoral component 5, bonecement may be used if necessary. Reference numeral 17 shown in FIG. 4represents a fibula.

The tibial tray 2 constituting the artificial knee joint 1 uses theslip-out prevention screw 11 as a slip-out prevention member and usesthe female thread portion 12 meshing with the slip-out prevention screw11 as a slip-out prevention member engaging unit. As the slip-outprevention member, a stopper ring an example of which is shown in FIG. 5may be used in addition to the slip-out prevention screw 11.

The stopper ring shown in FIG. 5 has a holding unit 20a at each end of acut-portion of a C-shaped concentric stopper ring. These holding units20a are formed by bending both the ends of the cut-portion in generallyan L-character shape and extending them inside of the C-shapedconcentric stopper ring. The holding units 20a can be nipped by a pairof pliers or the like having narrow tips from the outside. Therefore,the diameter of the stopper ring 20 can be made smaller easily.

FIG. 6 shows an example of a tibial tray using this stopper ring 20 asthe slip-out prevention member.

The tibial tray 22 shown in FIG. 6 has the same outer appearance as thetibial tray 2 shown in FIG. 2 except that the length of a projection 22bformed on the lower surface of a plate unit 22a constituting the tibialtray 22 is shorter than that shown in FIG. 2. Each projection 22b of thetibial tray 22 has a through hole unit 23 passing through the projection22b in the longitudinal direction thereof. A ring groove 24 is formed inthe circumferential inner wall of the through hole unit 23 at apredetermined position, the outside in the radial direction of thestopper ring 20 being engageable with the ring groove 24. This groove 24corresponds to the slip-out prevention member engaging unit.

Near at the lower end portion (the end on the insertion side into thetibia) of the through hole unit 23, a screw stopper 26 is formed forstopping a fixation screw 25 at the position where the head 25a of thefixation screw 25 is lower than the upper end of the through hole unit23, when the fixation screw 25 is inserted into the through hole unit23. The groove 24 is formed in the inner wall of the through hole unitat the position higher than the screw stopper 26.

The tibial tray 22 shown in FIG. 6 is mounted, for example, in thefollowing manner. First, similar to the tibial trader 2 shown in FIG. 1,a predetermined area of the proximal end of the tibia is ground to forma flat surface, a predetermined number of holes for inserting theprojections 22b and stems (not shown) of the tibial tray 22 thereintoare formed in the flat surface area and thereafter, the projections 2band stems 2c are inserted into these holes to align the position of thetibial tray 22. Next, each fixation screw 25 is inserted into thethrough hole unit 23 formed in the projection 22b and threaded into thetibia, until the head 25a of the fixation screw 25 abuts against thescrew stopper 26 or until a desired amount of gap is formed between thehead 25a of the fixation screw 25 and the screw stopper 26, to therebyfix the tibial tray 22 to the tibia 15. Thereafter, the two holdingunits 20a formed on the stopper ring 20 are nipped and squeezed with apair of pliers or the like having sharp tips to reduce the diameter ofthe stopper ring 20. In this state, the stopper ring 20 is inserted intothe through hole unit 23 and fitted into the groove 24. With the stopperring 20 being fitted into the groove 24, a slip-out prevention of thefixation screw 25 is ensured. A combination of the stopper ring 20,groove 24, and fixation screw 25 is preferably arranged so as to leave adesired amount of gap between the bottom surface of the stopper ring 20and the top surface of the fixation screw 25.

With the stopper ring 20 being fitted in the groove 24, even if thefixation screw 25 once threaded into the hard tissues (tibia) isspontaneously rotated and moved back outside, this motion is stopped bythe stopper ring 20 and the fixation screw 25 cannot be moved backoutside. Accordingly, after the tibial tray 22 is mounted on the tibia,loosening of the tibial tray 22 is not likely to be formed by thespontaneous rotation and back motion to outside of the fixation screw25. The stopper ring 20 fitted in the groove 24 can be easily removed bynipping and squeezing the two holding units 20a from the outside with apair of pliers or the like having sharp tips to reduce the diameter ofthe stopper ring 20. Reference numeral 25b shown in FIG. 6 represents ahexagonal hole in order to allow a rod spanner Of a hexagonal crosssection to enter this hole and rotate the fixation screw 25.

As the slip-out prevention member, caulking projections 28 examples ofwhich are shown in FIG. 7 may also be used. These caulking projections28 are formed upright on the head 29a of a fixation screw 29 asindicated by phantom lines (one-dot chain lines) in FIG. 7. As theslip-out prevention member engaging unit for these caulking projections28, a ring groove or a recess formed in the inner wall of the throughhole unit may be used. After the slip-out prevention screw 29 isthreaded into hard tissues to a predetermined depth, these caulkingprojections 28 are caulked (bent outward) as indicated by solid lines inFIG. 7 to insert the end portions of the caulked projections into theslip-out prevention member engaging unit (groove or recess).

With the end portions of the caulked projections 28 being inserted intothe slip-out prevention member engaging unit (groove or recess), even ifthe fixation screw 29 once threaded into the hard tissues (tibia) isspontaneously rotated and moved back outside, this motion is stopped bythe caulked projections 28 and the fixation screw 29 cannot be movedback outside. Although four caulking projections 28 are shown in FIG. 7,the number of caulking projections 28 is not limited to four, but thenumber may be selected as desired in a range from four to six caulkingprojections.

As described so far in connection with the embodiments of the invention,the medical substituting element for hard tissues of this invention isnot limited only to the tibial tray of an artificial knee joint of anunicompartmental replacement type. As described earlier, the inventionrelating to a medical substituting element for hard tissues isapplicable to various types of medical substituting elements for hardtissues capable of being fixed to hard tissues with fixation screws,including a fixation metal jig (bone fixation plate) for surgicaltreatment of a fracture, a augmentation material (a device with a boss)for an artificial ligament or an artificial tendon, an artificial boneof the type which is attached to the bone with fixation screws, a tibialtray, a tibial component and a femoral component of an artificial kneejoint, an acetabular component of an artificial hip joint, a tibialcomponent and a talar component of an artificial ankle joint, aglenoidalcomponent of an artificial shoulder joint, a humeral component and aulnar component of an artificial elbow joint, an artificial bone head,and the like. The overall shape of the medical substituting element forhard tissues can be selected as desired depending upon each application.

The artificial joint of this invention may be such an artificial jointhaving partially at its constituent the medical substituting element forhard tissues of this invention, and is not limited only to theartificial knee joint of the unicompartmental replacement type. Theinvention relating to the artificial joint is applicable to varioustypes of artificial joints having a constituent capable of being fixedto hard tissues with fixation screws, including an artificial knee joint(unicompartmental replacement type, total replacement type,non-constraint type, hemi-constraint type, constraint type, hinge type,hemi-hinge or rotation-hinge type), meniscus bearing type, tibialplateau type, and the like), an artificial hip joint, an artificialshoulder joint, an artificial ankle joint, an artificial elbow joint, anartificial joint with an artificial bone, and the like. The overallshape thereof can be selected as desired depending upon eachapplication.

What is claimed is:
 1. A medical substituting element for hard bodytissues comprising:a member made of a medically inorganic solid materialhaving one or a plurality of through hole units into which fixationscrews are inserted, each of the one or plurality of through hole unitsincluding an integral sleeve protecting from said member; a fixationscrew for each of said one or plurality of through hole units; a screwstopper surface formed in said sleeve for stopping said fixation screwinserted into said through hole unit at a position where a head of saidfixation screw lies below an upper end of said sleeve; a slip-offprevention member, a slip-off preventing member engaging means forengaging said slip-off prevention member, said means formed in an innerwall of said sleeve at a position higher than said screw stoppersurface; wherein in use, and after said fixation screw is inserted intosaid sleeve and is threaded into hard body tissues, said slip-offprevention member is engaged with said slip-off prevention memberengaging means to ensure slip-off prevention of said fixation screw. 2.A medical substituting element for hard body tissues according to claim1, and wherein said fixation screw passes through said sleeve along alongitudinal direction of said sleeve.
 3. A medical substituting elementfor hard body tissues according to claim 1, wherein said slip-offprevention member engaging means comprises a ring groove formed in aninner circumferential surface of said sleeve, and said slip-offprevention member comprises a stopper ring.
 4. A medical substitutingelement for hard body tissues according to claim 1, wherein saidslip-off prevention member engaging means comprises a ring groove orrecess formed in an inner circumferential surface of the sleeve, andsaid slip-off prevention member comprises a caulking projection formedon the head of said fixation screw.
 5. A medical substituting elementfor hard body tissues according to claim 1, further comprising one or aplurality of stems formed on a side of said member adapted to abutagainst the hard body tissues.
 6. A medical substituting element forhard body tissues according to claim 1, wherein said member comprises atibial tray or a tibial component of an artificial knee joint.
 7. Amedical substituting element for hard body tissues according to claim 1,wherein said sleeve reaches the cortex of the tibia when in use.
 8. Amedical substituting element for hard body tissues according to claim 1,wherein said member comprises a bone fixation plate, a reinforcingmember for an artificial ligament or an artificial tendon, an artificialbone of the type which is attached to a bone with fixation screws, afemoral component of an artificial knee joint, an acetabular componentof an artificial hip joint, a tibial component or a tatar component ofan artificial ankle joint, an acetabular tegmen side component of anartificial shoulder joint, a humeral component or a ulnar component ofan artificial elbow joint, or an artificial bone head.
 9. An artificialjoint having a first constituent member mounted on a ground surfaceformed on one bone constituting a joint and a second constituent membermounted on a ground surface formed on the other bone constituting thejoint, said first and second constituent members abutting against eachother directly or via a slide member, or being coupled together by amember with an open/close mechanism, whereinsaid first or secondconstituent member comprises:one or a plurality of through hole unitsinto which one or a plurality of respective fixation screws areinserted; a screw stopper for stopping said fixation screw inserted intosaid through hole unit, said screw stopper spaced vertically below anupper edge of said first or second constituent member but above an upperedge of said fixation screw; a slip-off prevention member; a slip-offprevention member engaging unit formed in an inner wall of said throughhole unit for engagement by said slip-off prevention member, whereinafter said fixation screw is inserted into said through hole unit andthreaded into hard body tissues, said slip-off prevention member isengaged with said slip-off prevention member engaging unit to ensureslip-off prevention of said fixation screw.
 10. An artificial jointaccording to claim 9, wherein said first or second constituent memberhas a projection formed on its lower surface, and said through hole unitextends through said projection along a longitudinal direction of saidthrough hole unit.
 11. An artificial joint according to claim 9, furthercomprising one or a plurality of stems formed on a side of saidartificial joint abutting against the hard body tissues.
 12. Anartificial joint according to claim 9 used for an artificial knee jointwherein said first constituent member is mounted on a ground surfaceformed on a shinbone and said second constituent member is mounted on aground surface formed on a thighbone,and wherein said first constituentmember comprises said one or a plurality of through hole units intowhich fixation screws are inserted.
 13. An artificial joint according toclaim 12, wherein said through hole unit extends through a projectionformed on a lower surface of said first constituent member, said throughhole unit extending through said projection along a longitudinaldirection, and wherein said projection reaches the cortex of the tibiawhen in use.
 14. An artificial joint according to claim 9, wherein theartificial joint is an artificial hip joint, an artificial joint with anartificial bone, an artificial shoulder joint, an artificial anklejoint, or an artificial elbow joint.